The field of the present invention relates generally to shape memory alloy (SMA) actuators. More particularly, the field of the present invention relates to a spatially distributed actuator film wherein a plurality of SMA actuator elements together with associated control and driver circuitry are deposited on a thin, flexible substrate using very large scale integrated circuit (VLSI) techniques.
The basis for a conventional steerable element such as a steerable catheter incorporating a shape memory alloy actuator is the ability of certain special alloys to undergo a micro-structural transformation from an austenitic phase at high temperature to a flexible, so-called martensitic phase at a lower temperature. One of the more common and useful alloys is a 49:51 composition of titanium and nickel (TiNi). The temperature at which the phase transition occurs is referred to as the activation temperature. For the foregoing composition, this is approximately 70.degree. C. In the low temperature region, the SMA actuator is soft and exhibits a Young's modulus of 3,000 MPa. In this state, the shape memory alloy may be readily deformed up to 5% in any direction without adversely affecting its memory properties.
Once heated just beyond the activation temperature, a phase transformation from the soft, pliable martensite to harder, inflexible (6900 Mpa) austenite, the parent phase, takes place. That is, if the shape memory alloy material is not excessively deformed or is not over-constrained, it attempts to reorganize its structure to a previously "memorized" shape. If permitted to cool, the shape memory alloy becomes soft again and may be mechanically deformed to begin another cycle. The mechanical deflections produced by activating the memorized state can produce useful work if suitably configured. Although the recovery deflections may be small (5%), the recovery forces can range from in the neighborhood of 35 tons per square inch or more for linear contractions. Thus, the recoverable energy is considerable.
Any shape may be programmed into an SMA actuator element by physically constraining the piece while heating it to the proper annealing temperature. TiNi alloys are commercially available in sheet, tube and wire forms and can have a wide range of transformation temperatures.
A memory transformation of an SMA element is dependent upon temperature. However, the rate of deformation is dependent on the rate of cooling and heating. Therefore, the rate at which temperature changes take place dictates the maximum speed at which the SMA actuator can operate. As with all mechanical designs, there is a tradeoff. A faster actuating SMA actuator must be heated and cooled faster, thereby consuming more power and generating a larger amount of wasted heat.
It is known to use shape memory alloy actuators in conventionally steerable elements such as catheters. One such application, U.S. Pat. No. 4,543,090, describes a conventional steerable and aimable catheter using shape memory alloy as the control elements. Conventional steerable devices using SMA elements are severely limited in dexterity. Movement is limited to a single plane. Also, the SMA element must be mechanically deformed to begin another cycle.
Thus, in conventional applications, each shape memory element must be coupled to at least one other shape memory element. When one of the elements is heated, it is returned to its original position by the other memory element. This enables controlled motion, however only in a plane. The motion is limited to at most, two degrees of freedom per joint.
Conventional steerable devices such as catheters incorporating SMA actuators as control elements have considerable disadvantages. The joints must be made unduly large and cumbersome because an opposite force is always needed to return the SMA actuator element to its martensitic shape after transformation from the parent phase. Complex linkages are required in order to rotate such a steerable device. For example, the range of maneuverability is severely limited by the linkages which are necessary to return the SMA actuator element to its martensitic shape after it has been activated to assume its programmed shape.
Conventional steerable devices using shape memory alloys have a further disadvantage in that they are relatively large and have a severely constrained lower limit beyond which size reduction is not economically feasible. The relatively large size is due to the need for control arms, linkages or other elements which are necessary to return the shape memory actuator to its initial state. This severely constrains the geometry of such a conventional steerable device.
Conventional steerable devices incorporating shape memory alloys lack the dexterity and precise control necessary to maneuver into very small, geometrically complex spaces. This is due to the need for control arms or oppositely disposed elements for mechanically returning the actuator to a first position after it has been activated to its programmed state.
Conventional steerable devices using SMA actuators are often too slow for many medical applications where quick, dexterous movement is critical. The large size of conventional steerable devices using SMA elements requires an increased amount of current in order to produce the activation temperature needed for a quick transition from the martensitic state to the programmed or "memorized" austenitic phase. A conventional SMA actuator consumes a great deal of power, thus dissipating a large amount of heat. This necessarily slows down the cooling to the activation threshold and thereby slows down the transition from the austenitic state back to the martensitic state, resulting in a slower acting device.
What is needed is a steerable device which is capable of unrestricted yet highly precise and dexterous maneuvers in three-dimensional space. It would be advantageous to eliminate the need for control arms, linkages, or other extraneous means for returning conventional shape memory alloy elements to a first position after deactivation and the transition from the parent phase back to the martensitic state. Such control linkages increase the size of the device, increase power requirements and slow the dissipation of heat, resulting in a slow acting device.
What is also needed is a steerable device capable of unrestricted articulation in three dimensions, and which can be scalable for providing increased dexterity and maneuverability in very small, geometrically constrained areas which are presently inaccessible to conventional steerable devices.